Phototransistor having a non-homogeneously base region

ABSTRACT

In a phototransistor which comprises an emitter, a collector and a base, base portions 20 and 21 are made unequal in impurity density, by which minority carriers of optically excited carriers are stored in the high impurity density regions 20 of the base and majority carriers are permitted to easily pass through the low impurity sensity regions 21 of the base, and voltages of the high and low impurity density regions are coupled together. A very high-sensitivity and high-speed operation can be achieved.

This application is a continuation, of application Ser. No. 610,301,filed Apr. 30, 1984, now abandoned.

TECHNICAL FIELD

The present invention relates to a high-speed and high-sensitivitysemiconductor device and, more particularly, to a phototransistor havinga nonhomogeneously doped impurity base region.

BACKGROUND OF THE INVENTION

Conventional p-n-p or n-p-n bipolar type phototransistors which have auniform or gradually-graded base structure have defects of lowsensitivity and low operating speed because of their high baseresistance and large collector-base and emitter-base capacitances.

FIG. 1 shows an example of the conventional bipolar phototransistor, 4being an emitter electrode, 1 an emitter region, 5 a base electrode, 2 abase region, 6 a collector electrode and 3 a collector region. 10 is alight input. The base electrode 5 may be made floating in some cases. 7is an insulating material such as an SiO₂ film or the like.

As shown in FIG. 1, in the portion of the base region 2, the distributedbase resistance R₁ between the emitter junction and the collectorjunction is high, because the emitter and collector are low in impuritydensity. Also, the distributed base lead-out resistance R₂ is high,because the base is low in impurity density and small in base thickness,and further, there are other parasitic resistances. Therefore, the valueof the base resistance of the bipolar phototransistor, including theseparasitic resistances, is very large.

The ratio of the density, ΔIn, of an amplified electron current flowingbetween the emitter and collector of the phototransistor of the n-p-nstructure to the total current density I by incident light, that is, thecurrent amplification factor, ΔIn/I, is given by the following equationin the case where the incident light intensity is very low and a darkcurrent component is also small. ##EQU1##

Here, D_(n) and D_(p) are diffusion coefficients of electrons and holes,L_(p) is the diffusion length of holes, W_(b) is the thickness of thebase, n_(e) is the impurity density of the emitter and P_(b) is theimpurity density of the base.

Eq. (1) is just the injection ratio of the emitter junction, I_(n)/I_(p), and this means that the higher the injection ratio of thetransistor is, the more its current amplification factor increases.I_(n) is the total emitter current and I_(p) a hole current.

To increase the current amplification factor of the conventional bipolarbase phototransistor, it is required to raise the impurity concentrationof the emitter region, and to reduce the impurity concentration of thebase region and to decrease the thickness of the base region. However,the reduction of the impurity concentration and the thickness of thebase region causes an increase in the parasitic base resistance.Therefore, this modification is not desirable.

Next, consider the operating speed of the phototransistor. The timeconstant for its rise and fall is given substantially by the followingequation: ##EQU2##

Here, +Φ_(eb) is the diffusion potential between the base and theemitter. The time constant decreases with a decrease in L_(p) /D_(p),but the current amplification factor diminishes according to Eq. (1). Adecrease in the thickness W_(b) of the base will decrease the timeconstant and increase the current amplification factor. The value of+φ_(eb) is the diffusion potential which is dependent upon the impuritydensities of the base and the emitter, and the reduction of thisdiffusion potential will cause a decrease in the time constant. Further,the value of +φ_(eb) decreases with a decrease in n_(e) or P_(b), but inorder to prevent the reduction of the current amplification factor, itis necessary only to decrease P_(b).

For the reasons given above, in order to raise the current amplificationfactor of such a conventional phototransistor of bipolar structure tospeed up its operation, there is no choice but to decrease the thicknessW_(b) of the base to thereby reduce its impurity density. As describedpreviously, however, this will increase the base resistance to imposelimitations on the performance of the transistor, providing only veryunsatisfactory results. At present, a p-i-n photodiode, an avalanchephotodiode and so forth are widely used as photosensing elements, butthe diode with two terminals is defective in that it is not sufficientlyisolated from the succeeding stage. The avalanche photodiode calls for arelatively high voltage (several tens of volts) and has a seriousdrawback of some amount of avalanche multiplication noise.

Accordingly, the present invention has for its object the providing of ahigh-speed and high-sensitivity phototransistor which is free from theabovesaid defects.

SUMMARY OF THE INVENTION

The present invention is directed to a phototransistor which comprisesan emitter, a collector and a base. The impurity concentration of thebase region is made nonhomogeneous in a plane perpendicular to a currentflow between the emitter and collector. Photogenerated carriers, whosetypes of conductivity is the same as that of the base, are stored in thehigh impurity concentration regions of the base region. Majoritycarriers, which are injected from the emitter region, and photogeneratedcarriers, whose conductivity type is different from the abovesaidphotogenerated carriers are permitted to easily pass through the lowimpurity concentration regions of the base region. The high and lowimpurity regions of the base region are made electricallycapacitive-coupled together. With such an arrangement, thephototransistor of the present invention is able to perform ahigh-sensitivity and high-speed operation which is free from thedrawbacks of the conventional bipolar phototransistor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a conventional bipolarphototransistor;

FIGS. 2A to D are diagrams showing the cross-sectional structures ofembodiments of the present invention with various base structures;

FIGS. 3A to C illustrate other embodiments of the present invention;

FIGS. 4A and B show examples of operation circuits of embodiments of thepresent invention which employ a common emitter and a common base,respectively;

FIGS. 5A to C illustrate examples of operation circuits of embodimentsof the phototransistor of the present invention in which a capacitor isconnected to the base, the emitter and the collector, respectively;

FIGS. 6A and B show examples of operation circuits in which a series anda parallel circuit of a capacitor and a resistor are connected to thebase electrode from the outside, respectively, to provide for increasedfunction; and

FIGS. 7A to D show operation circuits in which a series and a parallelcircuit of a resistor and a capacitor are connected to the emitter andthe collector from the outside, respectively.

BEST MODE FOR CARRYING OUT THE INVENTION

With reference to the drawings, the present invention will hereinafterbe described in detail. To obtain a high-sensitivity and high-speedphototransistor, it is necessary to decrease the base resistance and thecapacitance around the base.

The present invention utilizes, for the reduction of the baseresistance, the fact that the base resistance can be decreased to a verylow level by increasing the impurity density only in required regions ofthe base, instead of distributing its impurity density uniformlythroughout it. Low impurity density regions of the base are made tofacilitate the injection thereinto of monority carriers from theemitter. This can be achieved because the diffusion potential φ_(eb)between the abovesaid high impurity density regions of the base and theemitter is higher than the diffusion potential between the low impuritydensity regions of the base and the emitter region. The carrierinjection efficiency of the emitter essentially rises without theinjection of carriers into the emitter region from the low impuritydensity regions of the base. The two split base regions are capacitivelyconnected to each other, reducing the effective base resistance.

Upon irradiation by light, holes of the resulting electronhole pairs arestored in the high impurity density base regions, and electrons passthrough the low impurity density base regions and flow to the n regionof the collector. The holes charge the high impurity density baseregions positive to lower the diffusion potential φ_(eb) between thebase and emitter by ΔV_(eb), facilitating the flowing of many freeelectrons from the emitter into the low impurity density base regions.In consequence, as is evident from Eq. (1), the current amplificationfactor by light increases to a very high level as compared with thatobtainable with an ordinary bipolar phototransistor.

FIG. 2A illustrates an embodiment of a semiconductor device which is thephototransistor of the present invention. A description will be givenfirst of its structure. Reference numeral 11 indicates an n-type highimpurity density emitter region, 12 an n-type relatively low impuritydensity emitter region, 13 an n-type relatively low impurity densitycollector region, 14 an n-type relatively high impurity densitycollector region, 15 an emitter electrode, 17 a collector electrode and18 a surface protective film, which is an insulating film as of SiO₂.

Reference numeral 16 designates a base electrode. The base region is aregion defined between the emitter and the collector and is comprised ofp-type high impurity density regions 20 and p-type low impurity densityregions 21, and the geometrical configuration of the high impuritydensity regions 20 is larger than that of the low impurity densityregions 21.

A description will be given of the operation of this transistor. Withthe provision of such a base region, as described previously, when theoptical input 10 is applied, the holes are stored in the high impuritydensity base regions 20 and the electrons are injected from the emitterthrough the relatively low impurity density regions 21 to therebyincrease the current amplification factor for light, and the presence ofthe high impurity density base regions 20 makes the base resistance muchlower than in the case where the base is formed only by the low impuritydensity regions 21, so that the switching time also becomes very short.Further, since the relatively low impurity density regions are insertedbetween the emitter and the base and between the base and collector, thecapacitance between the emitter and the base and the capacitance betweenthe base and the collector decrease, contributing to the furtherenhancement of the switching characteristic.

As described above, according to the present invention, the baseresistance is reduced, the injection efficiency from the emitter is highand the capacitances between the base and the emitter and between thebase and the collector are small; therefore, a high-sensitivity andhigh-speed phototransistor can be achieved.

FIGS. 2B to D illustrate other embodiments, in which the bases havemodified shapes. In FIG. 2B the base-emitter junction is flat and the p⁺regions 20 of the base are formed to project out towards thebase-collector junction. In FIG. 2C the p⁺ regions 20 are formed toproject out towards the emitter unlike in FIG. 2B. FIG. 2D shows anexample in which the base is formed to the same thickness.

The phototransistor shown in FIG. 2A is fabricated in the followingmanner: The n⁻ layer 13 having an impurity density of 10¹² to 10¹⁶ cm⁻³is epitaxially grown 10 μm thick by a vapor growth method through usingSiCl₄ and H₂ gas. Next, the SiO₂ film 18 is formed on the epitaxiallayer, and by the diffusion of boron through a mask, the p⁺ regions 20having a high impurity density exceeding 10¹⁹ cm⁻³ are formed to a depthof 1 μm. The epitaxial growth is carried out again, thereby growing then-type high resistivity layer 12 of substantially the same resistivityas that of the region 13 to a thickness of about 3 μm. During theepitaxial growth the low impurity density p layers 21 are formed. Thehigh impurity density n⁺ layer 11 of the emitter, which has a valuelarger than 10¹⁹, is formed by diffusing phosphorus through a selectivediffusion method with the SiO₂ film. Next, chemical etching is performeddown to the p⁺ regions 20 of the base, exposing the p⁺ base regions 20.The electrode regions of the base and the emitter are exposed byoxidation and a mask method, and then aluminum is vapor-deposited onboth sides of the substrate assembly in a high vacuum. The top surfaceis subjected to selective etching to form the emitter and baseelectrodes. The bottom surface serves as the collector electrode.

To prevent the occurrence of lattice distortions in the P⁺ regions 20 ofthe base and then n⁺ region 11 of the emitter, it is desirable to adopta method which adds the Group IV elements, such as, for example,germanium or tin. If the carrier life is short, the rate of the carriersreaching the collector from the emitter drops, so that it is necessaryto minimize the mixing of a heavy metal which acts as a carrier killer,and it is preferable to effect gettering of heavy metals at the finalstage of the manufacturing process. The formation of the P⁺ layer of thebase can be accomplished by an ion implantation method, or a methodusing boron-doped polycrystalline silicon as a diffusion source.

FIGS. 3A to C illustrate other embodiments of the present invention. Inthese embodiments the low impurity density region of the base is formedby a high resistivity P⁻ region so as to raise the injection efficiencyfrom the emitter.

Reference numeral 20 indicates an n⁺ collector region (exceeding 1×10¹⁸cm⁻³), 21 a P⁻ base region of a low impurity density (10¹² to 10¹⁶cm⁻³), 22 high impurity density (higher than 1×10¹⁹ cm⁻³) P⁺ regions ofthe base, 23 n⁺ emitter regions (higher than 1×10¹⁹ cm⁻³) and 24, 25 and26 emitter, base and collector electrodes, respectively. Referencenumeral 27 in FIG. 3A to FIG. 3C indicates SiO₂ film. In order to reducethe base resistance, the base electrodes are interconnected.

FIG. 3B shows an embodiment of a structure in which etching is effecteddown to the high impurity density P⁺ regions of the base for thepurposes of raising the breakdown voltage between the base and theemitter, decreasing the capacitance between the emitter and the base andincreasing the area of irradiation by light. The etching down to the p⁺base regions can be accomplished by chemical etching, chemical etchingutilizing the anisotropy of crystals, a method using silicon nitride andoxide films, or plasma etching.

FIG. 3C is a top plan view of a phototransistor assembly in which anumber of such phototransistors of the embodiment shown in FIG. 3A or Bare arranged and in which the base and emitter electrodes are eachformed in a comb-shaped interdigitated pattern.

FIGS. 4A and B shows, by way of example, how to use the phototransistorof the present invention, A being a common emitter structure and B acommon base structure.

Reference numeral 30 identifies the phototransistor shown in FIG. 2 or3, 31 a variable external base resistor connected to the base electrode,32 a base-emitter power source, 33 a load resistor R_(L) of thecollector, 34 a collector-emitter power source, 35 an output terminaland 10 an optical input.

By virtue of its structure which minimizes the base resistance, thephototransistor of the present invention permits adjustment of itseffective base resistance over a very wide range through the variablebase resistor 31 connected to the base electrode and, consequently,possesses that feature of setting the value of variable base resistor 31in accordance with a request from a photodetector circuit which isunobtainable with the conventional bipolar phototransistor. FIG. 4Bshows an example which employs a common base structure in contrast to acommon emitter structure as shown in FIG. 4A.

FIGS. 5A, B and C illustrate embodiments of the phototransistor of thepresent invention in which capacitors are connected to the base, emitterand collector electrodes, respectively. By connecting capacitors 41, 42and 43 to the respective electrodes of the phototransistor 30, it ispossible with the phototransistor of the present invention to store anoptical signal resulting from the optical input 10 in the capacitorconnected to each electrode. It is needless to say that thephototransistors of the embodiments of FIG. 5, which are equipped withsuch a function, can be applied to a one transistor/one pixel typerandom access image sensor for optical information and various otherimage sensors having an optical information storage function.

FIGS. 6A and B illustrate embodiments of the phototransistor of thepresent invention in which a capacitor 41 and a resistor 51 areconnected to the base. The CR time constant of the base by the capacitor41 and the resistor 51 provides for extended function of thephototransistor.

In the same way, FIG. 7A and FIG. 7B illustrate other embodiments of thephototransistor of the present invention in which a capacitor 42 and aresistor 52 are connected to the emitter. In the same way, FIG. 7C andFIG. 7D illustrate other embodiments of the phototransistor of thepresent invention in which a capacitor 43 and a resistor 53 areconnected to the collector.

It is a matter of course that the phototransistor of the presentinvention can be formed to have a p⁺ -p⁻ -n⁺ -n⁻ -p⁺ structure which isreverse in conductivity from the aforementioned one.

The material for the phototransistor of the present invention is notlimited specifically to silicon but may also be germanium, of GaAs, GaP,AlAs and InP which are the Group III-V compound semiconductors, ofGa_(1-x) AlAs and the like which are mixed crystals thereof, or theGroup II-VI compound semiconductors.

AVAILABILITY FOR INDUSTRIAL USE

By virtue of its structure, the phototransistor of the present inventionpermits minimization of its base resistance to thereby raise the carrierinjection efficiency from the emitter into the collector through thebase and, consequently, it is able to perform a high-sensitivity andhigh-speed operation unobtainable in the past; therefore, thephototransistor or the invention takes the place of the conventionalphototransistor and is further available for wide industrialapplications.

I claim:
 1. A phototransistor comprising a semiconductor substratehaving a first surface and another surface opposite to the first surfaceand including an emitter electrode, a collector electrode and a baseelectrode, and an emitter region, a collector region, and a base regionhaving an impurity concentration distribution, and wherein thedistribution profile of the impurity concentration in the base region isnonhomogeneous in a base plane perpendicular to a current flow directionbetween the emitter region and collector region and includes high andlow impurity concentration region portions, the base region having afirst conductivity type and the emitter region and collector region bothhaving a second conductivity type which is the reverse of the firstconductivity type of the base region, photogenerated carriers whose typeof conductivity is the same as that of the nonhomogeneous base regionare stored in the high impurity concentration region portions of thebase region, majority carriers which are injected from the emitterregion and photogenerated carriers whose conductivity type is differentfrom the former mentioned photogenerated carriers are permitted toeasily pass through the low impurity concentration region portions ofthe base region, the high and low impurity concentration region portionsof the base region are electrically capacitive-coupled together;Thecurrent flow between the emitter region and collector region iscontrolled by the potential barrier-height relative to the emitterregion in the low impurity concentration region portions of the baseregion, the height of which is capacitively controlled by the potentialrelative to the emitter region of the high impurity concentration regionportions of the base region; The thickness in the direction from theemitter region towards the collector region of the high impurityconcentration region portions of the base region is no less than that ofthe low impurity concentration region portions of the base region in thesame direction so that no neutral region is formed; The emitter regionand the collector region have low impurity concentration region portionsformed in the vicinity of the base region which are reverse inconductivity type from that of the base region, and the emitter regionis formed in the first surface of the semi-conductor substrate and thecollector region is formed in the other surface of the semi-conductorsubstrate opposite to the first surface and the base region beingdivided into alternating high and low impurity concentration regionportions across a space between the first and the other surface andbeing buried in the semi-conductor substrate, and the base electrode iscontacted on a peripheral high impurity concentration region portion ofthe base region.
 2. A phototransistor comprising a semi-conductorsubstrate having a first surface and another surface opposite to thefirst surface and including an emitter electrode, a collector electrodeand a base electrode, and an emitter region, a collector region and abase region having an impurity concentration distribution, and whereinthe distribution profile of the impurity concentration in the baseregion is nonhomogeneous in a base plane perpendicular to a current flowdirection between the emitter region and collector region and includeslow and high impurity concentration region portions, the base regionhaving a first conductivity type and the emitter region and collectorregion both having a second conductivity type which is the reverse ofthe first conductivity type of the base region, photogenerated carrierswhose type of conductivity is the same as that of the base region arestored in the high impurity concentration region portions of the baseregion, majority carriers which are injected from the emitter region andphotogenerated carriers whose conductivity type is different from theformer mentioned photogenerated carriers are permitted to easily passthrough the low impurity concentration region portions of the baseregion, the high and low impurity concentration region portions of thebase region are electrically capacitive-coupled together;The currentflow between the emitter region and collector region is controlled bythe potential barrier-height relative to the emitter region in the lowimpurity concentration region portions of the base region, the height ofwhich is capacitively controlled by the potential relative to theemitter region of the high impurity concentration region portions of thebase region so that no neutral region is formed; and The emitter regionis formed in the first surface of the semi-conductor substrate andconnected to the low impurity concentration region portions of the baseregion at a first junction, and the collector region is formed in theother surface of the semi-conductor substrate and connected to the lowimpurity concentration region portions of the base region at a secondjunction, the low impurity concentration region portions of the baseregion are connected together and the high impurity concentration regionportions of the base region are intermixed with the low impurityconcentration region portions of the base region, the high impurityregion portions of the base region being diffused in and spaced alongbelow impurity concentration region portions of the base region, and abase electrode connected to each high impurity concentration regionportion of the base region with all of the base electrode beingconnected together.
 3. The phototransistor of claim 2, wherein the highimpurity concentration region portions of the base region are formed ina recessed surface of the semi-conductor substrate near the firstjunction.